Address set_player_privs footgun

[appgurueu]

Straightforward PR to get rid of the set_player_privs footgun. This effectively does three things:

• Document this more clearly: privs are a set, set_player_privs replaces the privs (unlike set_properties for example, which just changes properties).
• Add warnings for false values (modder probably meant to revoke), add warning for non-true values (code smell).
• Add a change_player_privs convenience wrapper so modders have a function they can use to grant/revoke privs without having to call get_player_privs / set_player_privs themselves. I think granting / revoking privileges is probably the most common use case.

How to test

• minetest.set_player_privs("singleplayer", {interact = "maybe"}) should warn about a non-true value.
• minetest.set_player_privs("singleplayer", {interact = false}) should warn about granting a privilege rather than revoking it.
• minetest.change_player_privs("singleplayer", {interact = 42}) should warn.
• minetest.change_player_privs("singleplayer", {fast = true, fly = false}) should grant fast and revoke fly.

[sfan5]

should probably be core.log("deprecated"

also I would leave off the "granting a privilege rather than revoking it" part

[appgurueu]

I went with grorp's suggestion.

[appgurueu]

By the way, it looks like log_level='deprecated' is not documented in lua_api.md, and it also seems to be slightly broken? (For example I get (at [C]:-1) when I execute this via dbg's /lua command). But that's out of scope for this PR. It's more appropriate than 'warning', though.

[sfan5]

By the way, it looks like log_level='deprecated' is not documented in lua_api.md,

Looks more like an internal engine helper to me. Please don't document it.

For example I get (at [C]:-1) when I execute this via dbg's /lua command

The C code hardcodes 2 as a stack depth, that's why.

[grorp]

LGTM otherwise